The forming of complex semiconductor devices often involves attaching two wafers together. In some designs the use of an adhesive or epoxy adversely impacts the performance of the product. In such design, wafer bonding is often used to attach wafers together.
Wafer bonding occurs when two wafer surfaces with sufficiently small total thickness variations are brought together at room temperature. The surfaces of the wafers are subjected to a plasma or chemical treatment to increase the density of bonding sites on the wafer surfaces. Van der Waals bonds are formed between the two surfaces, which can be annealed to form stronger covalent bonds.
Hydrophilic bonding occurs in an aqueous environment between oxide layers formed on the wafer surfaces. A plasma treatment creates holes for bonding with hydroxide ions separated from water molecules and hydroxide ions from opposite wafer surfaces form van der Waals bonds. Hydrophobic bonding occurs in an environment with little or no water directly between wafers. A plasma treatment creates free electrons for bonding with hydrogen ions to form van der Waals bonds.
These techniques for wafer bonding require exerting a large magnitude of force onto the wafers during the bonding process to prevent the wafers from becoming misaligned. In an arrangement with a backside illumination sensor, poor alignment can result in offset of a color filter in the backside illumination sensor, thereby altering the colors of the detected image. The large force in turn distorts the surface of the wafer. In an arrangement with a backside illumination sensor, wafer surface distortion reduces the image resolution of the sensor.